The present invention relates to a granular bed filtering arrangement and more particularly to a granular bed system for filtering and collecting particulate matter of very small size from gaseous streams. The invention also pertains to a process of separating entrained particulate material from a gaseous stream.
Granular bed filters have been used extensively to collect the dust from hot process exhaust systems. One type of granular bed filter used for this purpose is exemplified by those illustrated in Berz, U.S. Pat. No. 3,868,237 and No. 3,871,846. In such a filter, the dust-contaminated exhaust gas, after flowing through a cyclone-type separator, flows downwardly through a horizontally arranged granular filter bed in which the dust is entrapped. When the bed becomes dustladen, a backflush gas is blown upwardly through the bed to dislodge the collected dust, preferably in agglomerated chunks, and carry it into the cyclone for separation from the backflush gas. The backflush gas goes into the inlet, contaminated gas stream and thence to another granular bed filter of the system for cleaning
Granular bed filters of this type which employ a stationary bed and a backflushing step operate very satisfactorily for many applications, for example, in cleaning exhaust gas streams from cement manufacturing plants. However, this type of system also possesses certain inherent disadvantages which limit its application. For example, such a system is generally not suitable for separation of sub-micron particle sizes, since this requires use of a granular material having mesh sizes which are too fine to permit effective backflushing, i.e., the fine bed particles will be blown out of the bed during backflushing. It is impractical to simply increase the thickness of the bed, since this increases the pressure drop across the bed and results in an unacceptable increase in the operating costs for the system. Another limitation of this type of system is that it is capable of removing from the system only the dust which is agglomerated into chunks large enough to be separated by the cyclone. The non-agglomerated dust tends to simply build up in the system and ultimately requires some measure to remove it.
Another commercially available granular bed filter, e.g., of the type shown in U.S. Pat. No. 4,017,278 to Reese, employs a vertically contained granular bed which is generally continuously moving. This type of system obviates the need to have separate cleaning or backflushing phases. The movement of the bed, however, is a disturbing influence which interferes with the collection of the dust in the granular bed which would otherwise occur if the entire bed were quiescent. In addition, the granules at the extreme outer edges of the filter chamber do not travel vertically downward, as does the interior of the bed, because the constant refill does not allow a drain space between the louvers of the filter walls. External dedusting of the granular material removes the separated dust from the system.
Thus, there exists a need for a granular bed filter system which is capable of effectively removing sub-micron size particles and which is economical with respect to both initial installation and operation. Granular bed filters are, in general, energy inefficient.
There also exists a need for a granular bed filter system having an improved means for dedusting the spent granular filter media. Conventional methods, such as backflushing and the use of vibrating screens, involve disadvantages such as plugging, wear and high energy consumption.